It is known for two or more wireless short range radio-frequency devices, or Bluetooth (trademark) devices, to form a private network known as a piconet. A piconet comprises, a master device and up to a maximum (according to the Bluetooth standard) of seven active slave devices.
FIG. 1 is an example of the prior art. There is shown a piconet A which will be used as a basis for the embodiment of the invention as described below. The piconet A consists of a master device B, a maximum of seven slave devices C, and RF connections for transfer of information D between the master device B and the slave devices C. In the preferred embodiment the master device B is a known mobile telecommunications device which has a radio-frequency transmitter and receiver that complies to known Bluetooth specifications, but may be any suitable device which has wireless short range radio-frequency capabilities, for example but not limited to a personal portable computer, a watch, a Wibree® transmitter etc. The preferred embodiment of the slave devices C is a bluetooth tag, but may be any suitable device which has Bluetooth capabilities, for example but not limited to a personal portable computer, mobile phone, a dongle etc. The master device B and slave device C are ordinary Bluetooth devices with the standard two part architecture, comprising the controller E and the Bluetooth stack F. The controller E consists of the hardware such as the Radio Frequency Controller (RF), a link controller (LC) and a link manager (LMP). The Bluetooth stack F consists of the known standard communication protocols, such as L2CAP, RFCOMM, HCI etc. to communicate with the controller E. In a piconet A, the master device B can transmit data D to any slave device C, but a slave device cannot transmit data to another slave device. The slave devices C that form the piconet A are known as active, slave devices C that are known to the master device B, but do not form part of the piconet A are known as inactive or parked.
The use of piconets to form ad-hoc networks to transfer data between devices is well known, however there is currently no example of using a master device to maintain and monitor a portable piconet by measuring the distance between the master and slave devices. The Bluetooth standard does not specify a mechanism for calculating the separation of devices and as such it is impossible to perform a single calculation to calculate the distance between devices in a piconet that will work on all Bluetooth enabled devices. Furthermore, there are no examples of a portable device that is able to determine the bearings of a slave Bluetooth device with respect to the master device. The currently known methods for determining the bearings require triangulation between two or more fixed devices to determine the location of a portable slave device.